1. Field
The present application relates to optical metrology, and more particularly to optical metrology model optimization for repetitive structures.
2. Related Art
Optical metrology involves directing an incident beam at a structure, measuring the resulting diffracted beam, and analyzing the diffracted beam to determine various characteristics, such as the profile of the structure. In semiconductor manufacturing, optical metrology is typically used for quality assurance. For example, after fabricating a periodic grating structure in proximity to a semiconductor chip on a semiconductor wafer, an optical metrology system is used to determine the profile of the periodic grating. By determining the profile of the periodic grating structure, the quality of the fabrication process utilized to form the periodic grating structure, and by extension the semiconductor chip proximate the periodic grating structure, can be evaluated.
In optical metrology, an optical metrology model is typically developed to measure a structure. The optical metrology model can be expressed using optical metrology variables. In general, the greater the number of optical metrology variables that are allowed to float in developing the optical metrology model, the better the fitting of the measurements signal obtained to the simulated signal using the optical metrology model. However, increasing the number of optical metrology variables allowed to float also increases the amount of time needed to develop the optical metrology model. Additionally, in some cases, allowing too many optical metrology variables can produce erroneous measurements, due to correlation between the optical metrology variables. In some cases, floating correlated or insensitive optical metrology variables may also result in unstable and erroneous measurements.